1. Field of the Invention
The present invention relates to a method and apparatus of optically detecting a beam of light such as laser beam and the like. More particularly, the present invention concerns a photodetection apparatus suitable for use in such a rotating laser system as known in the fields of survey, construction and civil engineering and a method of photodetection therein.
2. Description of the Related Art
Photodetection techniques for measuring the intensity of light, the position of projected light and others have been broadly utilized in various fields.
FIG. 5 shows an example of rotating laser systems for performing the leveling in the survey, construction and civil engineering. Such a rotating laser system comprises a laser beam projecting device 2 and a photodetection device 1 for receiving the laser beam from the laser beam projecting device 2.
The laser beam projecting device 2 emits a laser beam L turned about a vertical axis 0 to define a reference plane P. The photodetection device 1 is positioned on a vertical stationary plane such as wall or pile and comprises at least first and second light receiving elements 11 and 12.
If the laser beam L from the laser beam projecting device 2 is projected only onto the upper light receiving element 11, this means that a reference mark 5 in the photodetection device 1 is positioned below the reference plane P defined by the laser beam L. This position of the laser beam causes "upward direction" indicators 3b and 4b to light on. Each of these indicators 3band 4b has an upward arrow indicating the necessary upward movement of the photodetection device 1. On the contrary, if the laser beam is projected only onto the lowerlight receiving element 12, this means that the reference mark 5 in the photodetection device 12 is positioned above the reference plane L. As a result, it causes "downward direction" indicators 3a and 4a each having an downward arrow to light on. The necessary downward movement of the photodetection device 1 is thus indicated. In addition, if the laser beam L is equally projected on both the upper and lower light receiving elements 11 and 12, it causes "center" indicators 3c and 4c to light on. This informs an operator that the reference mark 5 in the photodetection device 1 is brought into alignment with the reference plane P. The operator can obtain a leveling mark by drawing a line le on the wall or pile at the position of the reference mark 5.
Since the rotating laser device is frequently used out in the fields, the light receiving elements 11 and 12 will also receive disturbance rays as the sun's rays in addition to the laser beam L from the laser beam projecting device 2 which is used for measurement. When the sun's rays enter the light receiving elements 11 and 12, these elements will generate an output signal I.sub.O which includes normal signals I.sub.S from the laser beam L and DC noise signal components I.sub.N superposing on the normal signals, as shown in FIG. 6(a) which shows a graph having a horizontal axis representing time and a vertical axis showing signal output. Even if the rotating laser system is used indoors under the illumination of incandescent lamp or the like, normal signals are similarly superposed by DC noise signal components.
In order to overcome such a problem, the prior art photodetection device 1 comprises AC coupling circuits as composed of capacitor, which are connected with the light receiving elements 11 and 12, respectively. As shown in FIG. 6(b), the AC coupling circuits serves to remove DC noise signal components I.sub.N based on disturbance rays from the sun or incandescent lamp and to take only AC components I.sub.sc (hereinafter called "normal output signals") based on the normal signals I.sub.S from the laser beam L.
However, if any obstructive such as a person or motorcar moves across the optical path between the laser beam projecting device 2 and the photodetection device 1 during measurement or if the photodetection device 1 is rotated about its vertical axis during measurement or if the sun is blocked by cloud for a brief time, the amount of illumination rays entering the light receiving elements 11 and 12 from the sun or incandescent lamp will vary to have AC noise signal components I.sub.A, as shown in FIG. 6(a).
The AC coupling circuits in the prior art photodetection device 1 cannot the AC noise signal components I.sub.A. Therefore, AC noise signal components I.sub.AC will be provided to the photodectection device 1. As a result, the processing circuit will judge the AC noise signal components I.sub.AC as normal output signals I.sub.SC, as shown in FIG. 6(b). This results in errors in operation and measurement.
The present invention is directed to provide a photodetection method and apparatus which can prevent any error in operation and measurement from being created from the influence of disturbance rays from a source of pulsed light such as fluorescent lamp or the like.